Sensors often are used as sensor bridges, for example with four identical sensor elements coupled in a Wheatstone bridge configuration. Bridge circuits are supplied by a voltage or current and provide a differential output voltage. Examples include stress sensors, magnetoresistive sensors, and Hall plates and vertical Hall devices, among others.
A common problem with sensor bridges, however is offset error. Offest is the output signal in the absence of the physical quantity which the sensor should detect. For example, for Hall plates the offset is the output signal at zero applied magnetic field, and for stress sensors it is the output signal at zero mechanical stress. The origin of offset error typically is a slight mismatch between the sensor elements of the bridge. In other words, the “identical” sensor elements are not exactly identical. A typical mismatch is on the order of about 0.1% to about 1%, which means that although the four sensor elements have identical resistances they actually differ by about 0.1-1%.
Conventional approaches include, for Hall sensors, using the spinning current principle which uses different sensor elements in multiple clock phases to cancel any offset and enhance magnetic field proportional terms in the signals. This technique can be extended to more than two clock phases by reversing supply polarities and using more than four sensor elements. This technique, however, still results in a small offset error, referred to as the residual offset. The residual offset typically is on the order of about 30 micro-Tesla for Hall plates and about 0.5-1 mT for vertical Hall devices.
Therefore, there is a need for improved offset error compensation for sensors.